Noise and baseline disturbances in indirect UV detection in capillary electrophoresis

Abstract

In capillary electrophoresis (CE) with indirect detection, one is frequently confronted with low-frequency noise, occurring besides the instrumental noise that is dependent on the inherent qualities of the detector. Often wavy baseline disturbances, as well as disturbances resembling real peaks are observed. This non-detector noise type, which often shows repeating profiles, leads to a dramatic decrease in the quantitative reliability when using indirect detection. It was found that the non-detector noise originates from Joule heat production and non-uniform thermostatting along the capillary, which affect the electrophoretic transport process of ions in the background electrolyte (BGE) and consequently lead to a fluctuation of the visualizing ion concentration in the capillary. It is shown that disturbances are generated by so-called thermal nodes along the capillary, at the moment that the current is switched on. The number and mobilities of disturbances starting from each thermal node are equivalent to those of system zones, which are inherent to the BGE. The disturbances can reach the detector from the injection or the grounded side of the capillary. The magnitude of a disturbance is dependent on the thermal characteristics of the thermal node and on the composition of the BGE. Low-frequency noise, on the other hand, is generated from a multitude of small thermal nodes existing at the start of the electrophoresis or appearing newly during the CE process.